Details of connectivity :
Host : host1 (runs xen)
Guest : guest1 (runs under host1 xen server).
Host :host2 (different host on the nework)
To be able to ping from host2 to guest1
On host2:
#route add -host guest1 gw host1
You can mention the IP address or host name both works.
Now try pinging from Host2 to Guest1 !!
